A family of bacteriocin ABC transporters carry out proteolytic processing of their substrates concomitant with export

Abstract

Lantibiotic and non-lantibiotic bacteriocins are synthesized as precursor peptides containing N-terminal extensions (leader peptides) which are cleaved off during maturation. Most non-lantibiotics and also some lantibiotics have leader peptides of the so-called double-glycine type. These leader peptides share consensus sequences and also a common processing site with two conserved glycine residues in positions -1 and -2. The double-glycine-type leader peptides are unrelated to the N-terminal signal sequences which direct proteins across the cytoplasmic membrane via the sec pathway. Their processing sites are also different from typical signal peptidase cleavage sites, suggesting that a different processing enzyme is involved. Peptide bacteriocins are exported across the cytoplasmic membrane by a dedicated ATP-binding cassette (ABC) transporter. Here we show that the ABC transporter is the maturation protease and that its proteolytic domain resides in the N-terminal part of the protein. This result demonstrates that the ABC transporter has a dual function: (i) removal of the leader peptide from its substrate, and (ii) translocation of its substrate across the cytoplasmic membrane. This represents a novel strategy for secretion of bacterial proteins.